a b s t r a c tDeveloping models and algorithms to generate robust project schedules that are less sensitive to disturbances are essential in today's highly competitive uncertain project environments. This paper addresses robust scheduling in project environments; specifically, we address the discrete time/cost trade-off problem (DTCTP). We formulate the robust DTCTP with three alternative optimization models in which interval uncertainty is assumed for the unknown cost parameters. We develop exact and heuristic algorithms to solve these robust optimization models. Furthermore, we compare the schedules that have been generated with these models on the basis of schedule robustness.

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A common lament of the preventive maintenance (PM) crusaders is that production supervisors are often unwilling to lose valuable machine time when there are job waiting to be processed and do not assign high enough priority to PM. Maintenance activities that depend dynamically on system state are too complicated to implement and their overall impact on system performance, measured in terms of average tardiness or work-in-process (WIP) inventory, is dicult to predict. In this article, we present some easy to implement state-dependent PM policies that are consistent with the realities of production environment. We also develop polling models based analyses that could be used to obtain system performance metrics when such policies are implemented. We show that there are situations in which increased PM activity can lower total expected WIP (and overall tardiness) on its own, i.e., without accounting for the lower unplanned downtime. We also include examples that explain the interaction between duration of PM activity and switchover times. We identify cases in which a simple state-independent PM policy outperforms the more sophisticated state-dependent policies. Ó

We study an asynchronous optical packet (OP)-switching node equipped with a number of limited range (LR) wavelength converters shared per output link. We study both circular and noncircular LR-wavelength-conversion schemes. A wavelength conversion policy governs the selection of the outgoing wavelength for an OP if the incoming wavelength is in use. Through simulations, we show that the so-called far-conversion policy for which the OP is switched onto the farthest available wavelength in the tuning range, outperforms the other policies we studied. We point out the clustering effect in the use of wavelengths to explain this phenomenon. We also provide an approximate analytical method to find the packet-blocking probability in circular-type LR-wavelength-conversion systems. Based on the simulation results, the approximate method appears to lead to a lower bound for blocking probabilities for all the conversion policies we study. © 2007 Optical Society of America

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